1. Field of the Invention
This invention relates to an optical waveguide suitable for use in integration with a light emitting device and/or a light receiving device, its fabrication method, and an optical interconnection device using the waveguide as optical wiring.
2. Related Background Art
An optical waveguide is composed of highly-transparent material with a line width in a range between about 2 to 3 μm and about 20 to 30 μm. The waveguide can achieve a variety of functions, such as optical transmission, optical branching/combining, wavelength filtering, wavelength multiplexing/demultiplexing, and optical modulation of light intensity or phase. Therefore, the waveguide is expected to be widely used in the fields of optical information transmission, such as optical communication and optical interconnection, and information processing, such as optical memory.
As a light emitting device for performing optical transmission through the waveguide, there has been recently developed a surface emitting laser, which emits light perpendicularly to its substrate, and which has a low threshold, and can be readily arrayed. A light emitting diode (LED) is also well known as a light emitting device whose fabrication cost is low and which can be readily implemented. The configuration of such a surface light-emitting-type device is similar to that of a light receiving device, such as a photodiode, which originally receives light at its surface. Therefore, those surface light emitting and receiving devices can be suitably combined, and are hence expected to be applied to an optical interconnection that optically connects boards, modules in a board, and large scale integration (LSI) chips to each other.
For those reasons, there is an increasing expectation for a waveguide, which can be readily mass-produced, and freely formed not only on a glass, quartz, or resin substrate, but also on a Si wafer, a semiconductor-on-insulator (SOI), or a compound semiconductor wafer, such as GaAs or InP. Naturally, the following fundamental characteristics are also required to the waveguide. Its transmission loss is low, its insertion loss is low, and the waveguide can be readily coupled to the light emitting or receiving device.
In an active display unit and a reading unit which employs electroluminescence (EL), LED, or the like, there has also been proposed the use of the waveguide. Further, in a promising new field of optical information processing, such as a super-parallel processing/operation, the waveguide is expected to be used for transfer of optical information or signals.
Conventionally, as a typical waveguide and its fabrication method, there is a method in which metal ions are diffused on a substrate of SiO2, LiNbO3, or the like through a selective mask to form a portion with an altered refractive index in the substrate. There is also a method in which a protruding portion is formed on the surface of a substrate by etching. In addition, there is a method in which a SiO2 layer is formed on a Si wafer to fabricate a waveguide. There has recently proposed a method in which a resin, such as PMMA or polyimide, is coated on a substrate and a waveguide is formed on the substrate directly or using a patterned photoresist.
Thus, in the conventional methods, a waveguide is generally formed on the substrate by photolithography. As a result, a cross section of the waveguide is normally square or trapezoid. Further, when the waveguide is fabricated by etching, its side surface is considerably roughened, and hence, transmission loss occurs. Furthermore, when the waveguide is optically coupled to a light emitting or receiving device, the device must be precisely aligned with the end surface of the waveguide. Actually, it is typical to use one or two lenses between the device and the waveguide to improve the coupling efficiency. When the light emitting or receiving device is located above the waveguide, optical coupling is achieved by forming a 45-degree slanted mirror at the end surface of the waveguide, or forming a grating coupler. In those methods, however, the fabrication process is complex, and a sufficient coupling efficiency cannot be stably obtained.
An example of optical interconnection using a waveguide is disclosed in Japanese Patent Application Laid-Open No. 6(1994)-45584. In this structure, an optical wiring is formed in an integrated circuit by using the waveguide whose cross-sectional shape is square and whose end surface is a slantes surface for achieving an optical coupling to a light emitting or receiving device in a perpendicular direction. Accordingly, a lens for condensing propagating light needs to be additionally formed. Further, there is the limitation of a substrate for forming the waveguide therein since its fabrication method is conventional.